Health officials are working on a plan to monitor every person who returns to Maryland from countries stricken by Ebola, but the extent of the monitoring will depend on individual circumstances, the state’s health secretary said Wednesday.

All travelers who come into the U.S. from three Ebola-stricken West African nations will now be monitored for three weeks, the latest step by federal officials to keep the disease from spreading into the country.

Under pressure to select an Ebola “czar” to lead the U.S. response against the disease, President Barack Obama conceded Thursday it “may be appropriate for me to appoint an additional person” to head the administration effort.

More than 4,000 people have now died in the worst Ebola outbreak in history, and officials with the United Nations say the number of cases is doubling every three to four weeks in West Africa. Now the first human trials of an Ebola vaccine are underway.